Generally, thermal recording materials have, on a support, a heat-sensitive recording layer containing an electron-donating dye precursor (hereinafter referred to as a dye precursor) and an electron-accepting developer (hereinafter referred to as a developer). By application of heat to such thermal recording materials with a thermal print head, a thermal stylus, laser beam, etc., an instant reaction between the dye precursor and the developer occurs and thereby a recorded image is produced thereon. Such thermal recording materials have several advantages, for example, that recorded images are produced thereon with a relatively simple device ensuring easy maintenance and no noise generation, and therefore are widely used for measuring recorders, facsimiles, printers, computer terminals, label printers, ticket machines for passenger tickets or other tickets, and the like. Particularly in recent years, thermal recording materials are used as financial record slips such as receipts of gas, water, electricity and other bill payments, account statements issued from ATMs at financial institutions and various receipts, thermal recording labels or tags for point of sales (POS) system, etc. Thus, the application of thermal recording materials is rapidly expanding.
However, thermal recording materials used for the above-mentioned purposes have a problem. That is, when an image area printed on a thermal recording material comes into contact with plastics such as polyvinyl chloride, a plasticizer or other additives contained in the plastics will permeate the image area, which results in the reduction of the image density to an unreadable level. Therefore, there is a demand for thermal recording materials which are excellent in the stability of an image area against plasticizers (hereinafter referred to as plasticizer resistance). Moreover, recording devices used for the above-mentioned purposes are recently used in severer conditions, and accordingly, there is a strong demand for thermal recording materials which are excellent in the stability of an image area against hot and humid conditions after printing (hereinafter referred to as image stability against moisture and heat), in the stability of a non-image area (background area) against hot and humid conditions (hereinafter referred to as resistance to background fogging under moisture and heat), and in color developing ability at the time of printing after long-term storage (hereinafter referred to as retention of color developing ability), in particular, in the retention of color developing ability under hot and humid conditions.
Known thermal recording materials which are excellent in plasticizer resistance include, for example, thermal recording materials of Patent Literature 1 to 6, in which specific compounds having a phenylureido moiety in the molecule are used as a developer. These thermal recording materials are excellent in plasticizer resistance and image stability against moisture and heat, but are unsatisfactory in terms of the resistance to background fogging under moisture and heat. A known method for the improvement of the resistance to background fogging under moisture and heat is the use of acid-free paper as a paper support, but this method deteriorates the retention of color developing ability, in particular, the retention of color developing ability under hot and humid conditions. This is because the use of acid-free paper causes a developer having a phenylureido moiety in its molecule in the heat-sensitive recording layer to easily dissolve under hot and humid conditions and to spread into the paper support.
A thermal recording material having a waterproof heat-sensitive recording layer is described in, for example, Patent Literature 7, in which the waterproofing is achieved by the use of a hydrophobic resin emulsion in the heat-sensitive recording layer. However, this thermal recording material is inadequate in terms of the resistance to background fogging under moisture and heat, and unsatisfactory in terms of retention of color developing ability, in particular, the retention of color developing ability under hot and humid conditions.
Known methods for the improvement of the retention of color developing ability include a method using a high-melting-point alkyl ketene dimer as a sizing agent in the paper support, which is described in, for example, Patent Literature 8. However, alkyl ketene dimers, alkenyl succinic anhydrides, etc. may adversely affect the retention of color developing ability under hot and humid conditions, and there is a need for further improvement of such a method.